Result is Number1 + Number2
which should be preferred for portability.
The result is of type breal if any of the arguments is a breal, else float if any of the arguments is a float, else rational if any of the arguments is a rational. Only when both arguments are integers is the result an integer.
In coroutining mode, if Number1 or Number2 are uninstantiated, the call to +/3 is delayed until these variables are instantiated.
Success:
+(5, 2, 7).
+(5, -2.0, 3.0).
Fail:
+(1, 2, 7).
+(1, 2, 3.0).
+(5, 2, r).
Error:
+(A, 2, 6). (Error 4).
+(7 - 4, 2, 3). (Error 24).